Polyolefins, such as polyethylene (PE), are synthesized by polymerizing olefin, such as ethylene (CH2═CH2), monomers. Because it is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the properties polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
Olefin polymerizations are frequently carried out in a loop reactor using monomer, liquid diluent and catalyst, optionally one or more co-monomer(s), and hydrogen. The polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles which are suspended in the diluent. The slurry in the reactor is circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash vessel, where most of the diluent and unreacted monomers are flashed off and recycled.
Alternatively, the product slurry may be fed to a second loop reactor serially connected to the first loop reactor wherein a second polymer fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polymer product is a bimodal polymer product, which comprises a first polymer fraction produced in the first reactor and a second polymer fraction produced in the second reactor, and has a bimodal molecular weight distribution.
After the polymer product is collected from the reactor and the hydrocarbon residues are removed, the polymer product is dried, additives can be added and finally the polymer may be extruded and pelletized.
During the extrusion process ingredients including polymer product, optional additives, etc, are mixed intimately in order to obtain a compound as homogeneous as possible. Usually, this mixing is done in an extruder wherein the ingredients are mixed together and the polymer product and optionally some of the additives are melted so that intimate mixing can occur. The melt is then extruded into a rod, cooled and granulated, e.g. to form pellets. In this form the resulting compound can then be used for the manufacturing of different objects.
Loop reactors may be used for production of polyethylene with different characteristics. However, to avoid contamination of a subsequently produced polyethylene with a residue of a previously produced polyethylene, production equipment is cleaned or purged between production runs of different polymers. Cleaning minimizes the risk of contamination of the subsequently produced polyethylene product with the previously produced polyethylene product and aims to maintain the quality of the subsequently produced polyethylene batch, and/or prevent clogging of production equipment.
The purpose of the cleaning and purging operations is not only to avoid contamination of the produced products but also to optimize the conditioning of the polymer before production begins. As an example, purging of an extrusion apparatus is necessary to remove any material from previous runs adhering to the extrusion apparatus walls. If not removed, this material, subject to the heat of the extrusion process, degrades over time and can contaminate fresh polymer passing through the extrusion apparatus. This is in particular further true when the extrusion apparatus is used to consecutively extrude polymer products that have been prepared in the presence of different and incompatible polymerization catalysts.
A cleaning operation entails the participation of numerous operators. This work is not only very demanding in terms of labour, but also results in a significant down time of the reactor while cleaning operations are performed as cleaning procedures include extensive flushing of the reactor pipes, extensive physical effort with brushes, detergent and water, or even entirely reactor dismantling. Therefore, cleaning operations not only are time-consuming, but also are cost-ineffective.
In view of the above, there is a need in the art for an improved process for the consecutive production of different polyethylene products. In particular there is a need to streamline the transition of the production of one polyethylene product into a subsequent polyethylene product and to provide a production process which is less time consuming and generally more cost-effective.